Field of the Invention
The present invention relates to a variable valve lift apparatus. More particularly, the present invention relates to a variable valve lift apparatus of which operation stability is secured.
Description of Related Art
In general, an engine is provided with a combustion chamber for burning fuel to generate power, an intake valve to be opened and closed for selective supply of combustion chamber mixture gas, and an exhaust valve to be opened and closed for selective exhaust of exhaust gas.
The intake valve and the exhaust valve are opened and closed by a valve opening and closing device connected to a crankshaft for intaking the mixture gas and exhausting the exhaust gas, respectively.
A related art valve opening and closing device is operated by a cam of a fixed shape to open the valve to a fixed lift always, without changing timing or a valve lift of intake and exhaust. Consequently, appropriate intake and exhaust cannot be performed according to an operation state of the engine.
For an example, if a valve mechanism has been designed suitable for a low speed operation state, adequate mixture gas may not be supplied to the combustion chamber and exhaust from the combustion chamber may not be smooth in a high speed operation state when a relatively high output is required. And, if the valve mechanism has been designed suitable for the high speed operation state, excessive mixture gas may be supplied to the combustion chamber and an exhaust time period from the combustion chamber may be long excessively in the low speed operation state.
Recently, in order to solve such a problem, researches on a variable valve lift apparatus undergo, actively.
However, if the variable valve lift apparatus is operated only by a hydraulic pressure, smooth operation of the variable valve lift apparatus may become difficult as viscosity of the oil of the hydraulic pressure increases when an engine temperature becomes low which results in a low temperature of the oil.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the related art already known to a person skilled in the art.